The aim of this proposal is to test the hypothesis that the superficial layer of the Superior Colliculus (SC) play a role on attentional mechanisms. The activity of cells in cortical area V4 and cells in the SC increases when the stimulus presented in the receptive field is in the focus of attention (attended or target stimulus) as compared with the response to the same stimulus presentation if it is not in the focus of attention (unattended or distractor stimulus). The central question of this proposal is: does the activity of the SC modulate attentional activity of the area V4? or does area V4 modulate the attentional activity of the SC. This question can be addressed by testing the attentional modulation of the cellular activity of extrastiate area V4 during SC micro stimulation. The increase of the response of cells in area V4 to an unattended stimuli (distractor) concomitant to the SC microstimulation strongly suggests that collicular activity is one of the sources for extrastriate attentional modulation. The link between Collicular activity and cortical attentional modulation reinforces the hypothesis that the Superior Colliculus act as an attentional filter. Many cortical and sub-cortical disorders led to attentional impairment in humans. The understanding of the attentional mechanisms in non-human primates should give a model that can be tested on attentional impaired human patients.